A novel core-shell nanocomposite was prepared by coating carbon onto aluminum nanoparticles (ANPs). As a typical energetic material, this nanocomposite absorbs the heat energy prior to ignition and combustion. Molecular dynamics simulations were used to elucidate the phase-transition mechanism of the nanocomposite upon heating and to analyze migration at the core-shell boundary. The results show that the critical melting point of carbon-coated ANPs (8 nm in diameter), at which the material transitions from solid to liquid, is approximately 1050 K. The interatomic potential energy of nanocomposites is lower than that of aluminum cores. Therefore, upon injection of thermal energy, the initial amorphous phase transforms into the liquid phase, which then diffuses from the surface to the core. Low interatomic potential energy leads to an earlier transition to the liquid phase. Furthermore, the core-shell ratio is the main factor affecting the critical melting point. The higher the core-shell ratio, that is, the thinner the carbon coating, the lower the melting point. Therefore, effectively controlling this parameter is an important indicator of ignition efficiency.